"Julia" is a sound recorded on March 1, 1999 by the U.S. National Oceanic and Atmospheric Administration (NOAA). NOAA said the source of the sound was unknown, but sufficiently loud to be heard over the entire Equatorial Pacific Ocean autonomous hydrophone array. It lasted for about 15 seconds, and its origin was in the equatorial Pacific Ocean at approximately 15°S 98°W.
The Bloop is the name given to an ultra-low frequency and extremely powerful underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The source of the sound remains unknown. The sound, traced to somewhere around 50°S 100°W, was detected several times by the Equatorial Pacific Ocean autonomous hydrophone array, which uses U.S. Navy equipment originally designed to detect Soviet submarines. According to the NOAA description, it "rises rapidly in frequency over about one minute and was of sufficient amplitude to be heard on multiple sensors, at a range of over 5,000 km." The NOAA's Dr. Christopher Fox does not believe its origin is man-made, such as a submarine or bomb, or familiar geological events such as volcanoes or earthquakes. While the audio profile of the Bloop does resemble that of a living creature, the source is a mystery both because it is different from known sounds and because it was several times louder than the loudest living animal, the blue whale.
The Train is the name given to an unidentified sound recorded on March 5, 1997 on the Equatorial Pacific Ocean autonomous hydrophone array. The sound rises to a quasi-steady frequency. The origin of the sound is unknown. Coordinates: 5°S 107°W
Slow Down is a sound recorded on May 19, 1997, in the Equatorial Pacific Ocean by the U.S. National Oceanic and Atmospheric Administration. The source of the sound remains unknown. The name was given because the sound slowly decreases in frequency over about 7 minutes. The sound was detected at 15°S 115°W. It was recorded using an autonomous hydrophone array. The sound has been picked up several times each year since 1997.
The Whistle is an unidentified sound recorded by the autonomous hydrophone deployed at a location in the Pacific Ocean with coordinates 8°N 110°W. It was recorded on July 7, 1997. Origin of the signal is unknown, and it was not detected on any other hydrophone.
Upsweep is an unidentified sound detected on NOAA's equatorial autonomous hydrophone arrays. This sound was present when PMEL began recording SOSUS in August, 1991. It consists of a long train of narrow-band upsweeping sounds of several seconds duration each. The source level is high enough to be recorded throughout the Pacific. The sound appears to be seasonal, generally reaching peaks in spring and fall, but it is unclear whether this is due to changes in the source or seasonal changes in the propagation environment. The source can be roughly located at 54° S 140° W, near the location of inferred volcanic seismicity, but the origin of the sound is unresolved.